I. Field of the Invention
The present invention relates to communications, and more particularly to improved channel gating in a wireless communication system.
II. Description of the Related Art
Multiple access techniques are designed to make efficient use of the limited radio frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a wireless communication device to a central base station is to allow the wireless communication device to send the data using spread spectrum techniques of CDMA. CDMA wireless technology, governed by Telecommunication Industry Association/Electronic Industry Association (TIA/EIA) Interim Specification-95 (IS-95) and TIA/EIA IS-2000 specifications, both of which are hereby incorporated by reference, employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M. K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type mobile station system, multiple signals are transmitted simultaneously on the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
Since multiple mobile stations and base stations transmit on the same frequency, power control is an important component of the CDMA modulation technique. A higher power output by a mobile station and/or base station increases its signal quality, but also increases the interference experienced by the other mobile stations and base stations in the system. In order to keep the mobile stations and base stations from transmitting at too much power, thereby decreasing system capacity, some form of power control must be implemented.
The mobile station can aid the base station in the control of the power on the forward link (from the base station to the mobile station) by feedback on the reverse link (from the mobile station to the base station). This is accomplished by either a power control message that is sent when appropriate thresholds are triggered or an erasure indicator bit on a reverse link frame that indicates the status of a previously sent forward link frame. The base station may then adjust its power level to the specific user accordingly. This is referred to in the art as slow forward link power control.
In fast forward power control, the mobile station sends power-up or power-down commands to the base station to adjust the amount of power used by the base station for that particular mobile station. This adjustment corrects the bit-energy-to-noise-density of the received signal based on the performance of the forward traffic channel on a frame-by-frame basis. As the environment for a particular mobile station changes, the bit-energy-to-noise-density required may change in order to achieve a given frame error rate. The mobile station would instruct the base station to power up or down as needed to meet the required frame error rate. This decrease in the required receive power allows greater transmission rates to be processed successfully, or conversely allows a larger number of simultaneous users to access the system.
A reverse link power control process is also used by the base station to adjust the mobile station power level output by transmitting a power-up or power-down command to the mobile station. This adjustment corrects the bit-energy-to-noise-density of the received signal based on the performance of the reverse traffic channel on a frame by frame basis. As the environment for a particular mobile phone changes, the bit-energy-to-noise-density required may change in order to achieve a given frame error rate. The base station would instruct the mobile station to power up or down as needed to meet the required frame error rate. This decrease in the required receive power allows greater transmission rates to be processed successfully, or conversely, the interference between a set of reverse link signals to be decreased.
Another power control process is the use of transmission gating of the fundamental channel during lower sub-rate transmission. A fundamental channel is transmitted on the reverse link (the link from the wireless communication device to the base station) to carry information over the reverse link to the base station. The fundamental channel can be used in one of four sub-rates; full rate, half rate, quarter rate and eighth rate. Variable rate speech encoding of the fundamental channel utilizing these various rates is a commonly used method for speech transmission that offers particular advantages in increasing capacity, while maintaining high quality of perceived speech. Variable rate speech encoding transmits frames at full rate during speech, and frames at eighth rate during pauses in speech, with half rate and quarter rate as transitional states. It has been found that a majority of traffic channel frames during human speech are eighth-rate frames. When the variable rates speech encoder is providing frames at less than maximum rate, power consumption may be lessened by gating one or more transmission amplifiers such that only parts of the frame are transmitted.
A transmission gating feature for the reverse link fundamental channel provides a measurable increase in radio phone battery power life and reduction in interference energy on the reverse link. However, there is a forward link reduction in capacity when reverse link gating is enabled. By operating at the exemplary 50% duty cycle for eighth-rate frames, the ability to maintain power control on the reverse link and forward link is affected. During gating, the network must revert to 400 Hz forward link power control as it does not know the rate of the call on the reverse link until the end of the frame. As a result, the base station must assume for purposes of forward link power control that every frame is potentially an eighth-rate frame and hence only 50% of the power control groups would be valid. Thus, the update rate is reduced by a factor of two to avoid such an error. For example, the usual 800 Hz power control mechanism is reduced to 400 Hz, or from 16 decisions per frame to 8 decisions per frame. This reduction in power control has an associated loss in capacity because under certain channel conditions the lower rate power control results in an increase in per link user power (reduction in capacity) and frame error rate for the eighth-rate frames. Simulations have shown that on average a network operator can expect a 10% capacity loss in a system where every user is gating on the reverse link during eighth-rate transmission. Therefore, there is a need for an improved technique of transmission gating on the fundamental channel that provides the ability to balance the mobile station power savings of gating against the impact on system capacity.